Thin film optical and quasi-optical devices, which feature small energy absorption and minimal incidental signal interaction within a predetermined bandwidth, have acquired a growing utility in significant emerging technologies. (See, for example, Yariv, "Guided-Wave Optics", Scientific American (January 1979) pp. 64-72.) Thin film polarizers, for example, are useful in, inter alia, ellipsometry (the measurement of optical properties of materials, medical diagnostic instruments and laser communications systems such as the CO.sub.2 communication system diplexer. The polarization of radiation, an essential function within a large variety of electromagnetic systems, increases the ability of the abovenamed systems to transmit and detect useful information.
A popular polarizer presently used in the spectral range of 10-200 microns consists of a pile-of-plates inclined at the Brewster angle. The material of the plates depends upon its transmission at the wavelength of interest. Selenium, silicon and polyethylene have served as plate materials for wavelengths of common interest. The pile-of-plates does not form a thin film element. Rather, it comprises a relatively bulky device, limited functionally to parallel beams and a relatively narrow acceptance angle. An improvement over the pile-of-plates is obtained by the use of the well-known wire grid polarizer. Since before 1900 it has been known that a naked grid of parallel wires reflects one polarization of incident radiation while transmitting the other, provided that the wavelength of the incident radiation is larger than the period of the grid. Newly developed techniques have overcome the structural inadequacies inherent in a grid of unsupported wires. The work of G. R. Bird and M. Parrish ("The Wire Grid as a Near-Infrared Polarizer", J. Opt. Soc. Am., Vol. 50, pp. 886-891) demonstrated the construction of a wire grid polarizer supported by a plastic substrate in 1960. M. Hass and M. O'Hara described IR polarizers prepared on substrates of polymethylmethacrylate and polyethylene plastics in 1965 ("Sheet Infrared Transmission Polarizer", Appl. Opt., Vol. 4, pp. 1027-1031). More recently, fabrication techniques applicable to the thin film wire grid polarizer were disclosed in U.S. Pat. No. 4,049,944 granted to Garvin, Yariv and Somekh for " Process for Fabricating Small Geometry Semiconductive Devices Including Integrated Components". The above-referenced patent is the property of the assignee herein.
It is known that the extinction ratio (polarization selectivity) of the wire grid polarizer is proportional to (i) the ratio of the wavelength of incident radiation to the spacing (period) of the wires of the grid and (ii) the ratio of the width of the grid elements to their period. Extinction ratios of about 500:1 can be achieved by the wire grid polarizer fabricated within the bounds of state-of-the art technology. (See above-referenced patent to Garvin, et al.) This ratio does not represent sufficient polarization purity for a variety of electro-optical filter applications and many of the uses described above. Practical limitations to the extent of polarization achievable with the wire grid polarizer arise from the difficulty of maintaining electrical isolation between individual grid members as their spacing (period) is decreased. The shorting of the wires reduces the transparency of the grid to the incoming radiation, introducing (or augmenting) the undesired reflection of the electric field vector component perpendicular to the lengths of the wires of the grid. The extinction ratio, defined as the ratio of the amount of energy reflected by the grid with elements (wires) aligned parallel to the direction of polarization of incident radiation to the amount of energy reflected by the grid with elements aligned perpendicular to the direction of polarization is thereby reduced and limited by the incompatability of state-of-the-art manufacturing processes with the above-referenced theory of the wire grid which dictates a continual decrease in element spacing relative to element width to increase the extinction ratio.